Research On Mechanism Of Flux Pump For Bitter-like High Temperature Superconducting Magnet

In the strong magnetic fields,superconducting magnets can transmit high-density DC current,leading to superconducting magnets being the optimum option for high-magnetic-field magnets.However,power supply technology is the bottle neck technology in the operation of superconducting magnets at persistent current mode(PCM).Compared with the traditional current leads,both ends of the current leads are lying between cryogenic temperature and room temperature environment,which can cause a large amount of heat transferring to the superconducting cryogenic container when turning on the power supply.Besides,the superconductor and the current leads are welded and the joint resistance will generate Joule heat and power loss,increasing cooling penalty.As a new type of power supply device,the flux pump can replace the conventional air-cooled current leads,and have advantages in supplying power to superconducting magnets with relatively high operating currents.In this paper,the second-generation high-temperature superconducting(HTS)tapes,ReBaCuO(Re refers to rare earth elements),are used to manufacture the Bitter-like HTS magnets,while flux pump is mainly composed of a pulsed current source(the output current source is a triangular wave,and the rising edge time is far less than the falling time in one cycle)and a solenoid,serving as a power supply for the Bitter-like HTS magnet.Therefore,it is necessary to probe into the flux pump magnetization mechanism of the Bitter-like HTS magnet,and provide the feedback for the application of the fully-superconducting high-field Bitter-like HTS magnet.Firstly,a new type of Bitter-like HTS magnet with a rectangular double-hole structure is proposed.Four magnetization methods are used to magnetize this type of Bitter-like HTS magnet,including field-cooling(FC)experiment,zero-field-cooling(ZFC)experiment,FC and inner flux pump magnetization,and ZFC and inner flux pump magnetization,combined with commercially multiphysics simulation software COMSOL to establish a 3D model using H-formulation to verify the experimental results.The obtained results show that the FC with inner flux pump magnetization have the largest trapped field,followed by FC magnetization,and then ZFC and inner flux pump magnetization.The trapped field is the smallest in the case of ZFC magnetization.When the background magnetic field was 30 mT generated by the racetrack magnet,the final trapped field was approximately 24.5 mT,proving that the rectangular Bitter-like HTS magnet with two holes have excellent trapping capacity,with 90%of its background magnetic field.Additionally,in the FC and ZFC experiments,the amplitude of the trapped field of the two holes of the rectangular Bitter-like magnet with two holes was the same.Under inner magnetization,the insertion of a solenoid in the left hole(called the left magnetization)can increase the trapped field,but the trapped field of the right hole remained unchanged.In the same way,under the magnetization of the right hole(called the right magnetization),the trapped field of the right hole increased,while the left hole did not change.The simulation results were in good agreement with the experimental results.The experimental and simulation results have important reference value for the study of HTS flux pump magnetization,including the amplitude of the triangular wave power supply,the power supply frequency,and the design of the magnetic flux pump magnetization.Secondly,a 3D finite element model was adopted the latest T-A formulas to simulate and verify the magnetization mechanism of the flux pump.As far as the magnetization mechanism of the magnetic flux pump is concerned,combining the"flux pinning effect" of the non-ideal second-generation high-temperature superconducting tape and the principle of electromagnetic induction of transformers and the theory of high-temperature superconducting "critical state model",the mechanism of Bitter-like HTS magnet using flux pump magnetization is fully analyzed.The research results showed that the use of a rapidly changing applied pulse field and Faraday’s law to generate an induced current,causing the superconducting magnet to trap the magnetic field;it is also the magnetization nature of other forms of magnetism,such as ZFC magnetization,which means that the trapped magnetic field of Bitter-like HTS magnet can be attributed to the remanence effect.For a relatively low trapped magnetic field,the magnetic permeability of the inner iron core inside the solenoid made the applied background magnetic field smaller and it is difficult to reach the outer edges of the superconducting magnet.According to the Critical State Model(CSM),the magnetic flux caused by various forces(including Lorentz force,pinning force and viscous force)inside the superconductor is also an explanation.Viscous force holds the responsibility of the superconducting magnet inside the boundary partly penetrating the magnet,and thus the magnetic field in the center of the superconducting magnet is greatly reduced.Finally,the superconducting ring prepared by the second-generation HTS tape is magnetized with inner and external magnetization,and the central trapped magnetic field of the superconducting ring was measured at the temperature of liquid nitrogen(77 K).The results showed that the trapped field of the superconducting ring in the inner magnetization was higher than the trapped field of the external magnetization;in the external magnetization,inserting the iron core into the superconducting ring can increase the trapped field;in the inner magnetization,the trapped field of superconducting ring in the closed-loop solenoid was higher than the value of the open-loop operation.This is because when the closed-loop solenoid was used to magnetize the superconducting ring,the leakage flux was smaller and the background magnetic field was relatively larger.